AGoldfishNotch-3Homologue Is Expressed in Neurogenic Regions of Embryonic, Adult, and Regenerating Brain and Retina

are thought to be involved in the regulation of cell fate decisions in a variety of embryonic tissues, particularly in the developing central nervous system (CNS) in Dro-sophila and vertebrates. In goldfish the CNS continues to develop and add neurons well into adulthood and has the capacity to regenerate new neurons. Using probes derived from Xenopus Notch to screen an adult goldfish retinal cDNA library, followed by 58 RACE, we isolated a partial cDNA for a goldfish Notch homologue, G-Notch. Sequence alignment supported assignment of G-Notch to the Notch-3 class. Northern blot analysis revealed a single transcript of.8 kb, and RNase protection assays indicated thatG-Notch is expressed in eye and brain but not muscle of adult goldfish. The spatiotemporal pattern of expression of G-Notch was defined from early embryonic stages to adulthood by in situ hybridization. Expression in the embryonic CNSwas localized to neurogenic regions and was downregu-lated in differentiated cell populations. In adult goldfish, expression persisted in and adjacent to the germinal zones in the retina and the brain. Weak expression was seen in scattered cells in the inner nuclear layer of the retina, which might include neurogenic stem cells. Follow-ing retinal lesions (puncture wounds or laser lesions restricted to photoreceptors in the outer nuclear layer), G-Notch was upregulated in proliferating cell popula-tions throughout the retina, in association with a general-ized mitogenic response. In the region of the laser lesion, where earlier studies have demonstrated that photorecep-tors are regenerating at 1–3 weeks following the lesion, G-Notch expressing cells were abundant in the outer nuclear layer. These observations suggest that retinal regeneration involves the re-expression of an important developmental signaling molecule in neuroepithelial cells resident in the differentiated retina. Dev. Genet